The LIBS technique is based on generating a plasma by focusing a pulsed laser beam on a material. The radiation emitted by the plasma is detected by the LIBS system, a spectrum containing a continuum emission as well as the spectral lines of the elements forming the material being obtained. It is possible to determine the concentrations of said elements in the material by analyzing the spectrum.
Due to the optical nature of the energy source with which the plasma is generated, the LIBS technique is applied at a distance, without contacting the material being analyzed. Furthermore, a prior material preparation is not necessary. The technique is applicable to a solid or liquid sample surrounded by a specific ambient gas or in vacuum, or by focusing the laser directly on a liquid or a gas which is contained in a recipient or which is flowing. These characteristics make the LIBS technique a versatile analysis method with applications such as steel analysis in melting furnaces, analysis in plastic recycling, control of laser cleaning process for cleaning the contaminated stone of a building, detection of metal contaminants in the environment, detection of explosives, chemical analysis in space missions, etc.
A method for determining the concentration of atomic species in gases and solids by means of LIBS technique has been described in application WO 97/15811. In this method, at least two emission intensities of an atomic species in the sample are measured during a time interval with selected delay and length of time, determining the temperature of the plasma and normalizing the intensities. The concentration of an atomic species in a sample is determined by calibrating the measured intensity using the emission intensity previously measured for that species, present in a known concentration in a sample for calibration, similar to the unknown sample.
This method has the drawback of requiring a prior calibration for each of the elements of interest. This involves having a set of samples for calibration that are similar to the sample to be analyzed and contain known concentrations for each of the elements of interest. To achieve precise calibrations, the amount of known concentrations for each element must be significant.
To prevent the need of reference samples and prior calibration in the LIBS technique, U.S. Pat. No. 6,657,721 describes a solution based on the existence of local thermodynamic equilibrium (LTE) in the plasma. The temperature of the plasma is obtained from the intensities of emission lines and the spectroscopic data thereof available in the literature. Once the partition functions of all the radiation-emitting species are calculated, the product of their concentration by a common experimental factor is obtained for each element. The elimination of the experimental factor for determining the absolute concentrations is carried out by normalizing each concentration with respect to the sum of all the concentrations.
One drawback of this method is that the temperature and the relative concentrations are obtained under the hypothesis that the spectral lines used are optically thin, the method not including a way to check if this hypothesis is complied with. In fact, this condition is not usually verified in laser-induced plasmas due to their high density, particularly for the intense lines that provide the best precision to the results. Another drawback is that the method is based on the consideration of a homogeneous plasma, particularly with a single temperature value. In reality, laser-induced plasmas are inhomogeneous, having gradients in the characteristic parameters such as temperature, electron density and the densities of the atoms and ions present in the plasma. Taking temperature measurements without spatial resolution, as proposed in this method, means that the values obtained are apparent, population-averaged along the line of sight, resulting in different values for neutral atoms and ions, the emission of which comes from different regions of the plasma. A third drawback of this approach is the absence of a calibration curve or graph which, once known, allows readily obtaining the concentrations of the elements from the measurement of the intensities of spectral lines.
The solution described in application US 2012/0029836 prevents the limitations from the hypothesis of an optically thin and homogeneous plasma by proposing an alternative method for measuring concentrations by means of LIBS technique without prior calibration. To that end, it is based on calculating the absorption coefficient of the plasma for the spectral regions of the lines of interest. The spectral radiance of the plasma is calculated from the absorption coefficient using analytical solutions for the radiative transfer equation. By means of repeatedly comparing the intensity and shape of the spectrum calculated with those of the measured spectrum, the temperature, electron density, the relative elemental concentration values and the width of the plasma are adjusted. The method initially considers a single zone representing a uniform plasma. If, after optimizing the parameters, the difference between the calculated spectrum and the measured spectrum is greater than a predetermined threshold value, the plasma is divided into an increasing number of zones, in steps of three zones, along the direction of observation, characterized by different temperatures and electron densities. The method continues increasing the number of zones until convergence.
One drawback of this method is that it does not contemplate the elimination of spectral lines which, as a result of having a high intensity and/or due to the high concentration of the emitting element in the sample, have a high degree of self-absorption in the plasma, which is hard to describe even with a model with several zones. The inclusion of these lines the spectrum of which is poorly described by the model will reduce the precision of the deduced concentrations. On the other hand, the method includes the determination of the parameters of the plasma and of the elemental concentrations as consecutive steps included in the process. Therefore, it does not allow separating the determination of compositions from the complex method of characterizing the plasma. The absence of a calibration curve or graph is also a drawback of this method.